The present invention relates to brake control valve arrangements in general, and more particularly to an arrangement of this type which controls the pressure of a hydraulic fluid which is supplied from an actuating pressure source into a brake actuating cylinder in dependence on the pressure developed in a control pressure source in dependence on the position of a brake actuating member, such as a brake pedal.
There are already known various constructions of control valve arrangements of this type which control the hydraulic brake pressure of at least one wheel brake actuating cylinder in dependence on the control pressure which is developed, for instance, in a master cylinder of a vehicle braking system. The conventional arrangements of this type usually include a housing having ports communicating with a low-pressure supply reservoir, and an actuating pressure source, such as a pressure accumulator. In some of the conventional constructions, a control piston is accommodated in the interior of the housing and is acted on in one direction by the control pressure. Then, a hollow valve member is received in the interior of the housing in alignment with the control piston and in the aforementioned one direction therefrom. The end of the valve member which is closer to the control piston cooperates with the close end portion of the control piston to form a first valve device which, when opened, establishes communication through a first passageway from the wheel brake actuating cylinder port to the low-pressure supply reservoir port of the housing. The other end of the valve member cooperates with the housing or with a component which is rigid with the housing proper and constitutes housing means therewith, to constitute a second valve device. When the second valve device is open, it establishes communication through a second passageway leading from the actuating pressure port to the brake actuating cylinder port of the housing. A compression spring acts on the end of the valve member which is remote from the control piston in a direction toward the control piston and toward a position in which the second valve device is closed.
One construction of a control arrangement of this type is disclosed in German Pat. No. 1,232,827. This conventional arrangement includes, in addition to the pressure accumulator, another pressure source constituted by a hydraulic pump which is operative for feeding pressurized hydraulic fluid to the brake actuating cylinders, either in addition to, or instead of, the pressure accumulator. The control pressure for closing the first valve device in order to interrupt the communication of the brake actuating cylinders with the supply reservoir, and for opening the second valve device in order to establish communication between the control pressure source and the brake actuating cylinders, is applied in a mechanical manner.
A particular disadvantage of this particular brake control arrangement is that, when operated in the pressure accumulator mode, a larger amount of control pressure is required during the initial phase of the braking operation for opening the second valve device than during the subsequent phases. The requirement for the relatively high level of the control pressure during the initial phase of brake operation necessarily results from the relatively high closing force of the compression spring which urges the second valve device towards its closing position and which has to be overcome before any pressurized fluid from the actuating pressure source is allowed to flow to the brake actuating cylinders. Accordingly, it is necessary to apply a relatively high force to the brake actuating member during the initial phase of braking action in order to render the brake operative. This conveys to the driver of the vehicle equipped with a hydraulic braking system incorporating such a brake control arrangement the impression that the brake has failed. In many cases, the driver will react to this perceived brake failure by increasing the pressure on the brake pedal or a similar brake actuating member. This will result in an overcompensation for the perceived inadequate brake behavior, so that the braking action will be too pronounced after the expiration of the initial phase of the braking operation. This is disadvantageous especially when the vehicle is being maneuvered at the same time.